Air/fuel ratio control apparatus for general-purpose engine

ABSTRACT

In an apparatus for controlling an air/fuel ratio of a general-purpose internal combustion engine using mixed fuel containing alcohol and gasoline and operated at a desired engine speed inputted by the operator while a throttle opening is regulated such that a detected engine speed converges to the inputted desired engine speed, a fuel injection amount for mixed fuel determined based on fuel injection amount characteristics is increased/decreased when a load is kept constant and the output air/fuel ratio (at which an output of the engine becomes maximum) is estimated. And the fuel injection amount is corrected based on the estimated air/fuel ratio, thereby enabling to estimate the output air/fuel ratio without using an expensive air/fuel ratio sensor and to feedback-control the fuel injection amount such that the engine is operated at the output air/fuel ratio.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to an air/fuel ratio control apparatus for ageneral-purpose internal combustion engine.

Description of the Related Art

Conventionally, as one type of air/fuel ratio control of an internalcombustion engine, air/fuel ratio feedback control is known which usesan air/fuel ratio sensor installed in an exhaust pipe to detect anair/fuel ratio of exhaust gas and corrects a fuel injection amount suchthat the detected air/fuel ratio converges to the stoichiometricair/fuel ratio, as taught, for example, in Japanese Laid-Open PatentApplication No. 2002-122035.

SUMMARY OF THE INVENTION

As to a general-purpose engine to be mounted on a machine such as agenerator and lawnmower, it is preferable to operate the engine with aspecific air/fuel ratio (so-called the “output air/fuel ratio”) that isricher than the stoichiometric air/fuel ratio such that the engineoutput becomes maximum. Therefore, an O₂ sensor (oxygen sensor) whoseoutput changes at the stoichiometric air/fuel ratio or thereabout is notsuitable for controlling the engine to operate at the output air/fuelratio. A wide-range air/fuel ratio sensor that produces an outputproportional to the air/fuel ratio enables such the engine control.However, since the wide-range air/fuel sensor is expensive, it isdifficult to provide such an air/fuel ratio sensor to a compact andinexpensive general-purpose engine from the cost viewpoint.

An object of this invention is therefore to overcome the foregoingproblem by providing an air/fuel ratio control apparatus for ageneral-purpose engine that can estimate the output air/fuel ratiowithout using an expensive air/fuel ratio sensor, and feedback-controlthe fuel injection amount such that the engine is operated at the outputair/fuel ratio.

In order to achieve the object, this invention provides in its firstaspect an apparatus for controlling an air/fuel ratio of ageneral-purpose internal combustion engine that is connectable to aload, having a desired engine speed input switch adapted for an operatorto input a desired speed of the engine, an engine speed detector thatdetects a speed of the engine, a throttle opening regulator thatregulates an opening of a throttle valve installed in an air intake pipeof the engine such that the detected engine speed converges to theinputted desired engine speed, a fuel injection amount calculator thatcalculates a fuel injection amount of the engine based on the detectedengine speed and the regulated throttle opening in accordance with fuelinjection amount characteristics, and an injector that injects fuelbased on the calculated fuel injection amount, wherein the improvementcomprises: a load determiner that determines whether the load connectedto the engine is kept constant; a fuel injection amount injectingincreasing/decreasing unit that increases/decreases the calculated fuelinjection amount to be injected when the load connected to the engine isdetermined to be kept constant; an air/fuel ratio estimator thatestimates an air/fuel ratio at which an output of the engine becomesmaximum based on the throttle opening regulated when the fuel injectionamount is fuel of the increased/decreased; and a fuel injection amountcorrector that corrects the fuel injection amount based on the estimatedair/fuel ratio.

In order to achieve the object, this invention provides in its secondaspect a method for controlling an air/fuel ratio of a general-purposeinternal combustion engine that is connectable to a load, having adesired engine speed input switch adapted for an operator to input adesired speed of the engine, an engine speed detector that detects aspeed of the engine, a throttle opening regulator that regulates anopening of a throttle valve installed in an air intake pipe of theengine such that the detected engine speed converges to the inputteddesired engine speed, a fuel injection amount calculator that calculatesa fuel injection amount of the engine based on the detected engine speedand the regulated throttle opening in accordance with fuel injectionamount characteristics, and an injector that injects fuel based on thecalculated fuel injection amount, wherein the improvement comprises thesteps of: determining whether the load connected to the engine is keptconstant; increasing/decreasing the calculated fuel injection amount tobe injected when the load connected to the engine is determined to bekept constant; estimating an air/fuel ratio at which an output of theengine becomes maximum based on the throttle opening regulated when thefuel injection amount is fuel of the increased/decreased; and correctingthe fuel injection amount based on the estimated air/fuel ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will be moreapparent from the following description and drawings in which:

FIG. 1 is an overall schematic view showing an air/fuel ratio controlapparatus for a general-purpose engine according to an embodiment ofthis invention;

FIG. 2 is a view showing details of a desired engine speed input knobshown in FIG. 1;

FIG. 3 is a view showing details of a map switching knob shown in FIG.1;

FIG. 4 is a block diagram showing operation of an electronic controlunit (ECU) shown in FIG. 1 in a functional manner;

FIG. 5 is a flowchart showing the operation of the ECU shown in FIG. 4;

FIG. 6 is a subroutine flowchart showing calculation of a feedbackcorrection coefficient referred to in the FIG. 5 flowchart;

FIG. 7 is a subroutine flowchart showing estimation of an outputair/fuel ratio referred to in the FIG. 6 flowchart;

FIG. 8 is a graph showing a principle of the estimation of the outputair/fuel ratio of FIG. 7;

FIG. 9 is a graph for explaining a fuel injection amount (Qmin)corresponding to the output air/fuel ratio of the FIG. 7 flowchart; and

FIG. 10 is a subroutine flowchart showing estimation of alcohol rate andforcible switching of fuel injection amount map in response theretoreferred to in the FIG. 5 flowchart.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An air/fuel ratio control apparatus for a general-purpose engineaccording to an embodiment of the present invention will now beexplained with reference to the attached drawings.

FIG. 1 is an overall schematic view showing an air/fuel ratio controlapparatus for a general-purpose engine according to the embodiment ofthis invention.

In FIG. 1, symbol 10 designates the air/fuel ratio control apparatus fora general-purpose engine 12 (shown in cross-section) 12. The apparatus10 has an Electronic Control Unit (ECU) 14 for electronicallycontrolling the engine 12. The ECU 14 comprises a microcomputer having aCPU, memory (EEPROM or non-volatile memory) and other components.

The engine 12 is a single-cylinder, four-cycle, air-cooled, OHV enginewith a displacement of, for example, 440 cc, and is a general-purposeengine to be mounted on a machine such as a generator, lawnmower andsnowplow which functions as a load when connected to the engine 12.

An air intake pipe 16 of the engine 12 is installed with a throttlevalve 18 and injector 20. Intake air sucked in and flowing in the airintake pipe 16 is regulated through the throttle valve 18 and fuelinjected from the injector 20 are mixed thereto. The thus-generatedair-fuel mixture is drawn into a combustion chamber 24 when an intakevalve 22 is opened and burned or exploded upon being ignited by a sparkplug 26, thereby driving a piston 28. The driven piston 28 rotates acrankshaft 32 through a connecting rod 30. The crankshaft 32 isconnected to the machine (load), and its rotation is transmitted to theload.

The crankshaft 32 is provided along its circumference with a pluralityof (i.e., eight) pulsar coils 34 and when the crankshaft 32 is rotated,a pulsar coil detector 36 installed at a position close to the outerperiphery of the pulsar coils 34 produces an output or signal (crankangle signal Pc) indicative of a position of the piston 28. The burnedair-fuel mixture is discharged to an exhaust pipe 40 when an exhaustvalve 38 is opened.

An electric motor (more precisely, a stepper motor; referred to as“throttle motor”) 42 is connected to the throttle valve 18.Specifically, the throttle motor 42 is connected to a rotary shaft ofthe throttle valve 18 through a speed reducer. The throttle motor 42 isoperated to regulate an opening of the throttle valve 18, i.e., athrottle opening.

The injector 20 is supplied with fuel stored in the fuel tank andpressurized by a fuel supply device including a fuel pump. The fuel ismixed fuel containing alcohol (i.e., ethanol) and gasoline.

A temperature sensor 46 is installed near a cylinder 44 and produces asignal indicating a temperature of the engine 12. The aforementionedinjector 20, spark plug 26, pulsar coil detector 36, throttle motor 42and temperature sensor 46 are electrically connected to the ECU 14.

The ECU 14 is connected with a desired engine speed input knob (switch)48 to be manipulated by the operator to input a desired speed of theengine 12, a map switching knob (switch) 50 enabled to be manipulated bythe operator to switch one from among sets of fuel injection amount maps(mapped data; characteristics) to another, and an indicator 52. Theindicator 52 comprises a lamp including an LED (Light-Emitting Diode) orthe like and located to be visible to the operator.

FIG. 2 is a view showing details of the desired engine speed input knob48 and FIG. 3 is a view showing details of the map switching knob 50.

The desired engine speed input knob 48 is made turnable (adjustable)continuously within a scale range between 1000 rpm and 5000 rpm. The mapswitching knob 50 is also made turnable to switch to select one fromamong maps (sets of characteristics defining the fuel injection amounts)corresponding to low, medium and high alcohol rates. The low alcoholrate represents a rate of about 25%, the medium alcohol rate a rate ofabout 50%, and the high alcohol rate a rate of about 75%, for instance.

The knobs 48 and 50 are installed to be pinched with operator's fingersand manipulated by the operator. The operator manipulates (turns) themap switching knob 50 to a position corresponding to an assumed(estimated) alcohol rate of the mixed fuel stored in the fuel tank.

Based on the detection signals of the pulsar coil detector 36 andtemperature sensor 46 and the input signals of the desired engine speedinput knob 48 and map switching knob 50, the ECU 14 controls the fuelinjection through the injector 20, the ignition timing through the sparkplug 26, the throttle opening through the throttle motor 42. Further itcontrols turning on/off of the indicator 52.

FIG. 4 is a block diagram showing the operation of the ECU 14 in afunctional manner. As illustrated, the ECU 14 has an engine speeddetection block (engine speed detector), a governor control block(throttle opening regulator), a fuel injection amount calculation block(fuel injection amount calculator), a feedback correction coefficientcalculation block and an ignition timing calculation block.

The engine speed detection block counts a time interval betweensuccessive ones of the crank angle signal Pc detected by the pulsar coildetector 36 to detect the engine speed NE (rpm).

The governor control block is inputted with the desired speed Nd of theengine 12 inputted through the desired engine speed input knob 48 andthe engine speed NE detected in the engine speed detection block, andcontrols the throttle opening such that the detected engine speed NEconverges at the desired engine speed Nd.

Specifically, when the detected engine speed NE is lower than thedesired engine speed Nd, it calculates a throttle opening command valueθth to increase the throttle opening using PID control laws and outputsthe same to the throttle motor 42 via the drive circuit (not shown). Incontrast, when the engine speed NE is higher than the desired enginespeed Nd, it calculates the throttle opening command value θth todecrease the throttle opening using the PID control laws and outputs thesame to the throttle motor 42

The fuel injection amount calculation block calculates a fuel injectionamount Qmap by retrieving one from among three maps (three sets ofcharacteristics) experimentally prepared beforehand for the mixed fuelof the low, medium and high (i.e., 25, 50 and 75%) alcohol rates by theengine speed NE (detected in the engine speed detection block) and thethrottle opening command value θth (indicative of the engine loadoutputted from the governor control block). In each of the three maps,the fuel injection amount Qmap is experimentally prepared beforehandunder ideal conditions (i.e., ambient temperature: 25° C., altitude: 0meter, humidity: 50%).

The fuel injection amount calculation block is inputted with a signalindicating a position (i.e., designated alcohol rate) from the mapswitching knob 50 designated by the operator, so that one of the fuelinjection amount maps corresponding to the designated position of themap switching knob 50 is selected and the fuel injection amount iscalculated by retrieving the selected fuel injection amount map by theaforesaid parameters.

The fuel injection amount calculation block is also inputted with theengine temperature Te detected by the temperature sensor 46 and based onthe engine temperature Te, corrects the fuel injection amount calculatedin accordance with the fuel injection amount map at the engine start.

The feedback correction coefficient calculation block calculates afeedback correction coefficient K and operates the indicator 52 based onthe calculated coefficient K. Specifically, the fuel injection amountQmap calculated in the fuel injection amount calculation block isforwarded to the feedback correction coefficient calculation block. Thefeedback correction coefficient calculation block calculates thefeedback correction coefficient K in the manner explained later. Thefeedback correction coefficient K is stored in the memory.

The calculated coefficient K is sent to the fuel injection amountcalculation block, where the fuel injection amount map is corrected orrebuilt by multiplying each fuel injection amount in the map by thecoefficient K.

Specifically, the fuel injection amount is again calculated inaccordance with the corrected fuel injection amount map and thecalculation result is outputted to the injector 20 as a final fuelinjection amount command value Qf (or the fuel injection amount issimply corrected by the coefficient). Based on the sent value Qf, theinjector 20 injects the fuel at the intake port.

The ignition timing calculation block calculates the ignition timingbased on the crank angle signal Pc sent from the pulsar coil detector 36and outputs the calculated ignition timing to the spark plug 26, via anignition device (not shown).

FIG. 5 is a flowchart showing the operation of the ECU 14, i.e., theapparatus 10. The illustrated program is executed by the ECU 14 when theoperating power to the ECU 14 is being supplied by a starter motor (orrecoil starter).

The program begins in S10, in which it is determined whether the mapswitching knob 50 was switched, i.e., whether it was changed from adesignated position at the last engine stop to another position.

When the result in S10 is affirmative, the program proceeds to S12, inwhich the feedback correction coefficient K stored in the memory isinitialized (explained later). Specifically, the coefficient K is set to1.0.

When the result in S10 is negative, the program proceeds to S14, inwhich the coefficient K stored in the memory is read to correct orrebuild the fuel injection amount map. Specifically, the fuel injectionamount map is corrected by multiplying the map values corresponding tothe designated position of the map switching knob 50 by the coefficientK. Then the program proceeds to S16, in which the indicator 52 is turnedon.

The program then proceeds to S18, in which it is determined whether theengine stall has happened. Specifically, when the engine 12 is startedby the starter motor (or recoil starter), it is determined whether theengine 12 was stopped without reaching the self-rotational speed.

When the result in S18 is affirmative, since it means that the fuelinjection amount is not appropriate, for example, by the reason that thefuel injection amount corrected by the coefficient K is not appropriate,the program proceeds to S12, in which the coefficient K is initialized.Also the indicator 52 is turned off.

When the result in S18 is negative, the program proceeds to S20, inwhich it is confirmed whether the engine 12 is started and warm-upoperation after the engine start has been completed. The determinationis made based on the output signal of the temperature sensor 46.

When it is confirmed that the warm-up operation has been completed, theprogram proceeds to S22, in which the feedback correction coefficient Kis calculated.

FIG. 6 is a subroutine flowchart showing the process.

The program begins in S100, it is determined whether the desired enginelspeed Nd within a predetermined range is inputted, i.e., whether thedesired engine speed input knob 48 is set within the range between 1000rpm and 3000 rpm.

When the result in S100 is affirmative, the program proceeds to S102, inwhich it is determined whether the detected engine speed NE is equal toor almost equal to the desired engine speed Nd continuously for apredetermined time period, i.e., whether the engine speed NE remainswithin a range of plus and minus 200 rpm of the desired engine speed Ndconsecutively for 5 seconds.

When the result in S102 is affirmative, the program proceeds to S104, inwhich it is determined whether the throttle opening (precisely, thethrottle opening command value θth) is equal to or less than apredetermined throttle opening and a change amount of the throttleopening (precisely, the change amount of the throttle opening commandvalue θth) is equal to or less than a predetermined change amountcontinuously for the predetermined time period.

To be specific, it is determined whether the throttle opening is equalto or less than an opening of 30% (when the full throttle opening isdefined as 100%) and the change amount of the throttle opening remainswithin a range of plus and minus 1% for 5 seconds.

When the result in S104 is affirmative, the program proceeds to S106, inwhich it is determined that the load connected to the engine 12 is keptconstant.

The program next proceeds to S108, in which the fuel injection amountQmap is calculated. Specifically, the amount Qmap is calculated based onthe engine speed NE and throttle opening command value θth detectedunder a condition where the load connected to the engine 12 is constantin accordance with the fuel injection amount map (which is the map whosevalues have not been corrected in S14). More specifically, the amountQmap is calculated by retrieving one of the maps by the parameters.

Then the program proceeds to S110, in which an output air/fuel ratio isestimated.

FIG. 7 is a subroutine flowchart showing the process.

Before explaining this flowchart, a principle of the output air/fuelratio estimation will be explained first.

FIG. 8 is a graph showing the principle. The abscissa indicates theair/fuel ratio A/F and a dashed line in the graph represents the outputcharacteristics of the engine 12 relative to the air/fuel ratio A/F.Generally, the engine output becomes maximum at a specific air/fuelratio (i.e., the so-called “output air/fuel ratio”) on the richer sidethan the stoichiometric air/fuel ratio (A/F=14.7). In other words, theengine output decreases as the air/fuel ratio is changed to the leaneror richer side from the output air/fuel ratio.

When the load connected to the engine 12 is kept constant and the enginespeed NE is the same as the desired engine speed Nd, the throttleopening command value θth is controlled to be constant. Under thiscondition, if the fuel injection amount is intentionallyincreased/decreased, i.e., the air/fuel ratio is changed, the engineoutput is changed accordingly so that the engine speed NE is alsochanged.

Consequently, it also changes the throttle opening command value θth. Asillustrated in FIG. 8, the throttle opening (i.e., command value θth)becomes minimum at the point corresponding to the output air/fuel ratio,and increases in the richer or leaner air/fuel ratio direction.

Thus, under the condition where the load connected to the engine 12 iskept constant and the engine speed NE is kept the same as the desiredengine speed Nd, the minimum value of the throttle value is determinedwhen the fuel injection amount is intentionally increased or decreased,so that the output air/fuel ratio can be estimated.

The output air/fuel ratio estimation in the FIG. 7 flowchart is nowexplained.

The program begins n S200, in which the throttle opening command valueθth is read, while the fuel injection amount is increased. Explainingthis more concretely, the fuel injection amount is increased by 5% persecond, and the throttle opening command value θth regulated with theincreased fuel injection amount is read every 100 milliseconds tocalculate an average of the throttle opening command values θth for 1second. The increased fuel injection amounts (thus injected) and theaverages of the throttle opening command values θth are sequentiallystored in the memory.

The program then proceeds to S202, in which it is determined whether thethrottle opening command value θth is increased by a prescribed value ormore, i.e., whether the throttle opening command value θth (average)since the fuel injection amount increase is increased by 10% or morecompared to that before the fuel injection amount increase. Explainingthis in FIG. 8, it amounts to determining whether the value θth hasreached a point a. When the result in S202 is negative, the programproceeds back to S200.

On the other hand, when the result in S202 is affirmative, the programproceeds to S204, in which the throttle opening command value θth isread while the fuel injection amount is decreased. This is done bydecreasing the fuel injection amount by 5% per second, and the throttleopening command value θth regulated with the decreased fuel injectionamount is read every 100 milliseconds to calculate an average of thethrottle opening command values θth for 1 second. The decreased fuelinjection amounts (thus injected) and the averages of the throttleopening command values θth are sequentially stored in the memory.

The program then proceeds to S206, in which it is determined whether thethrottle opening command value θth is increased by a prescribed value ormore, i.e., whether the throttle opening command value θth (average)since the fuel injection amount decrease is increased by 5% or morecompared to that before the fuel injection amount decrease. Itcorresponds to determining whether the value θth has reached a point bin FIG. 8. When the result is negative, the program proceeds back toS204.

When the result in S206 is affirmative, the program proceeds to S208, inwhich a fuel injection amount Qmin corresponding to the output air/fuelratio is calculated. To be precise, as shown in FIG. 9, theincreased/decreased injection amount of injected fuel and the throttleopening command value θth (average) at the time of the fuel injectionare plotted.

Next, the characteristics of the change of the throttle opening commandvalue θth are approximated as a quadratic curve with the least squaresmethod. Then, the minimum value of the throttle opening command valueθth in the approximated quadratic curve is determined and the fuelinjection amount Qmin corresponding to the minimum value of the valueθth is calculated. This fuel injection amount corresponding to theminimum value of the value θth is the aforesaid fuel injection amountQmin corresponding to the output air/fuel ratio.

Returning to the explanation on the FIG. 6 flowchart, the programproceeds to S112, in which the feedback correction coefficient K iscalculated. As shown in the drawing, the coefficient K is calculatedbased on a ratio of the fuel injection amount Qmap calculated in S108 tothe aforesaid fuel injection amount Qmin corresponding to the outputair/fuel ratio calculated in S208.

In other words, the coefficient K represents a degree of deviationbetween the fuel injection amount under the ideal condition (i.e., thecondition when the fuel injection amount maps are prepared) and the fuelinjection amount corresponding to the output air/fuel ratio in an actualcondition where the engine 12 is operated.

Returning to the explanation on the FIG. 5 flowchart, the programproceeds to S24, in which the fuel injection amount map is corrected orrebuilt, i.e., the mapped values are corrected by multiplying by thecoefficient K calculated in S112 (or the fuel injection amount retrievedfrom the map is simply corrected by the coefficient K).

The program proceeds to S26, in which the indicator 52 is turned on. Theindicator 52 is turned on for informing the operator of the fact thatthe fuel injection amount (map) is thus corrected appropriately by thefeedback correction using the output air/fuel ratio estimation.

The program proceeds to S28, in which the coefficient K calculated inS112 is stored in the memory. The stored coefficient K will be used forcorrecting or rebuilding the fuel injection amount (map) at the nextengine start as mentioned in S14.

Then the program proceeds to S30, in which the alcohol rate estimationand fuel injection amount map forcible switching is conducted.

FIG. 10 is a subroutine flowchart showing the process.

The program begins in S300, in which it is determined whether thefeedback correction coefficient K is equal to or greater than apredetermined value α. The predetermined value α is set to a valuegreater than 1.0, e.g., 1.2.

When the result in S300 is affirmative, the program proceeds to S302, inwhich it is determined whether the map switching knob 50 is at aposition of the high alcohol rate.

When the result in S302 is affirmative, the program proceeds to S304, inwhich the indicator 52 is blinked. That is, the affirmative result inS302 means that the coefficient K calculated for the fuel injectionamount map corresponding to the high alcohol rate is equal to or greaterthan 1.2 and in this case, it is estimated that the alcohol rate of themixed fuel is very high (i.e., 90% or more). In response to thisestimation, the indicator 52 is blinked to prompt the operator to feedgasoline.

When the result in S302 is negative, the program proceeds to S306, inwhich it is determined whether the map switching knob 50 is at aposition of the medium alcohol rate.

When the result in S306 is affirmative, the program proceeds to S308, inwhich it is estimated that the alcohol rate of the mixed fuel is high.The reason is that, since the affirmative result in S306 means that thecoefficient K calculated for the fuel injection amount map correspondingto the medium alcohol rate is equal to or greater than 1.2, the alcoholrate is estimated to be higher than the medium alcohol rate (i.e., to be75% or thereabout). Consequently, the program proceeds to S310, in whichthe fuel injection amount map is forcibly switched to that for the highalcohol rate.

When the result in S306 is negative, the program proceeds to S312, inwhich it is estimated that the alcohol rate of the mixed fuel is aboutthe medium level. This is because, since the negative result in S306means that the coefficient K calculated for the fuel injection amountmap corresponding to the low alcohol rate is equal to or greater than1.2, the alcohol rate is estimated to be the medium alcohol rateslightly higher than the low alcohol rate (i.e., to be 50% orthereabout). Consequently, the program proceeds to S314, in which thefuel injection amount map is forcibly switched to that for the mediumalcohol rate.

When the result in S300 is negative, the program proceeds to S316, inwhich it is determined whether the coefficient K is equal to or lessthan a predetermined value β. The predetermined value β is set to avalue smaller than 1.0, e.g., 0.8.

When the result in S316 is affirmative, the program proceeds to S318, inwhich it is determined whether the map switching knob 50 is at aposition of the high alcohol rate.

When the result in S318 is affirmative, the program proceeds to S320, inwhich it is estimated that the alcohol rate of the mixed fuel is aboutthe medium level. Specifically, since the affirmative result in S318means that the coefficient K calculated for the fuel injection amountmap corresponding to the high alcohol rate is equal to or less than 0.8,the alcohol rate is estimated to be the medium alcohol rate slightlylower than the high alcohol rate (i.e., to be 50% or thereabout).Consequently, the program proceeds to S322, in which the fuel injectionamount map is forcibly switched to that for the medium alcohol rate.

When the result in S318 is negative, the program proceeds to S324, inwhich it is determined whether the map switching knob 50 is at aposition of the medium alcohol rate.

When the result in S324 is affirmative, the program proceeds to S326, inwhich it is estimated that the alcohol rate of the mixed fuel is low.Explaining this, since the affirmative result in S324 means that thecoefficient K calculated for the fuel injection amount map correspondingto the medium alcohol rate is equal to or less than 0.8, the alcoholrate is estimated to be lower than the medium alcohol rate (i.e., to be25% or thereabout). Accordingly, the program proceeds to S328, in whichthe fuel injection amount map is forcibly switched to that for the lowalcohol rate.

When the result in S316 or S324 is negative, the program is terminatedwithout forcibly switching the fuel injection amount map.

As stated above, the embodiment is configured to have an apparatus (10)and a method for controlling an air/fuel ratio of a general-purposeinternal combustion engine (12) that is connectable to a load, having adesired engine speed input switch (knob 48) adapted for an operator toinput a desired speed of the engine (Nd), an engine speed detector (ECU14) that detects a speed of the engine (NE), a throttle openingregulator (ECU 14; governor control block) that regulates an opening(θth) of a throttle valve (18) installed in an air intake pipe (16) ofthe engine such that the detected engine speed (NE) converges to theinputted desired engine speed (Nd), a fuel injection amount calculator(ECU 14) that calculates a fuel injection amount (Qmap) of the enginebased on the detected engine speed (NE) and the regulated throttleopening (θth) in accordance with fuel injection amount characteristics,and an injector (20) that injects fuel based on the calculated fuelinjection amount, characterized by: a load determiner (ECU 14, S22,S102, S104) that determines whether the load connected to the engine iskept constant; a fuel injection amount injecting increasing/decreasingunit (ECU 14, S22, S110, S200-S204) that increases/decreases thecalculated fuel injection amount (Qmap) to be injected when the loadconnected to the engine is determined to be kept constant; an air/fuelratio estimator (ECU 14, S22, S110, S208) that estimates an (output)air/fuel ratio (Qmin) at which an output of the engine becomes maximumbased on the throttle opening regulated when the fuel injection amountis fuel of the increased/decreased; and a fuel injection amountcorrector (ECU 14, S24) that corrects the fuel injection amount based onthe estimated air/fuel ratio.

Specifically, in the engine control apparatus 10 having the governorcontrol block (throttle opening regulator) which controls the throttleopening such that the engine speed converges to the desired enginespeed, it is configured to intentionally increase/decrease the fuelinjection amount calculated in accordance with the fuel injection amountcharacteristics (set beforehand) under the condition where the engineload is kept constant, whereby the engine output, i.e., the engine speedis changed. Based on the throttle opening which is also changedaccordingly, the output air/fuel ratio is estimated and the air/fuelratio used to correct the above fuel injection amount characteristics.With this, it becomes possible to, without using an expensive air/fuelratio sensor such as the wide-range air/fuel ratio sensor,feedback-control the fuel injection amount such that the engine isoperated at the output air/fuel ratio.

In the apparatus and method, the air/fuel ratio estimator determines aminimum value of the throttle opening regulated when the fuel injectionamount is increased/decreased and estimates the (output) air/fuel ratiobased on the determined minimum value (ECU 14, S22, S110, S208). Withthis, it becomes possible to appropriately estimate the output air/fuelratio with simple structure.

In the apparatus and method, the fuel injection amount correctorcalculates a correction coefficient (K) based on a ratio of thecalculated fuel injection amount (Qmap) to a fuel injection amount(Qmin) at which the throttle opening is minimum and corrects the fuelinjection amount by the coefficient (ECU 14, S24). With this, it becomespossible to feedback-correct the fuel injection amount without anexpensive air/fuel ratio sensor such as the O₂ sensor and wide-rangeair/fuel ratio sensor.

The apparatus and method further include: an informing device (LED 52,ECU 14, S26) that informs an operator of a fact that the fuel injectionamount is corrected. With this, it becomes possible to inform theoperator that the engine is operated appropriately at the outputair/fuel ratio.

The apparatus and method further include: a correction coefficientstoring unit (ECU 14, S28) that stores the correction coefficient whenthe fuel injection amount is corrected by the coefficient.

With this, at the next engine start, since the fuel injection amount iscorrected or is calculated in accordance with the fuel injection amountcharacteristics corrected or rebuilt with the stored correctioncoefficient, the engine can be appropriately started.

In the apparatus and method, the load determiner determines that theload connected to the engine is kept constant when the detected enginespeed is within a predetermined range set based on the desired enginespeed continuously for a predetermined time period (ECU 14, S22, S102).With this, it becomes possible to appropriately determine that theengine load is kept constant, with simple structure.

In the apparatus and method, the load determiner determines that theload connected to the engine is kept constant when the regulatedthrottle opening is equal to or less than a predetermined opening and achange amount of the regulated throttle opening is within apredetermined change amount continuously for a predetermined time period(ECU 14, S22, S104).

Specifically, when the engine is operated at the idling speedcontinuously for a predetermined time period, it is determined that theengine load is kept constant. With this, it becomes possible toappropriately determine that the engine load is kept constant, withsimple structure.

In the apparatus and method, the fuel is mixed fuel containing alcoholand gasoline. With this, when the output air/fuel ratio changesdepending on a mixture rate of alcohol to gasoline, since the outputair/fuel ratio can be accurately estimated, the engine can beappropriately operated at the output air/fuel ratio regardless of themixture rate.

In the apparatus and method, the load comprises at least one of agenerator, a lawnmower and a snowplow, and the informing devicecomprises at least one of a lamp diode (52) and a buzzer.

It should be noted that, although, in response to the affirmativeresults in both S102 and S104, the engine load is determined to be keptconstant, the determination may be made when the result of either S102or S104 is affirmative.

It should also be noted that, although the map switching knob 50 has thethree positions to be designated, the number may be increased dependingon the number of the fuel injection amount maps. Also, the knob 50 maybe configured to be continuously changed and in this case, the fuelinjection amount between adjacent maps is compensated.

It should further be noted that, although the operator is informedvisually by lighting the indicator 52 or the like, an audible devicesuch as a buzzer could instead be used.

Japanese Patent Application No. 2010-029033 filed on Feb. 12, 2010, isincorporated by reference herein in its entirety.

While the invention has thus been shown and described with reference tospecific embodiments, it should be noted that the invention is in no waylimited to the details of the described arrangements; changes andmodifications may be made without departing from the scope of theappended claims.

What is claimed is:
 1. An apparatus for controlling an air/fuel ratio of a general-purpose internal combustion engine that is supplied with mixed fuel containing alcohol and gasoline and stored in a fuel tank and that is connectable to a load, comprising a desired engine speed input switch adapted for an operator to input a desired speed of the engine, an engine speed detector that detects a speed of the engine, a throttle opening regulator that regulates an opening of a throttle valve installed in an air intake pipe of the engine such that the detected engine speed converges to the inputted desired engine speed, a map switch that selects one of a plurality of maps corresponding to respective rates of alcohol contained in the mixed fuel stored in the fuel tank, the respective rates of alcohol being greater than 0%, the map switch having a plurality of selectable positions corresponding to the respective rates of alcohol and selecting the one of the plurality of maps in response to a selected position, a fuel injection amount calculator that calculates a fuel injection amount of the engine based on the detected engine speed and the regulated throttle opening in accordance with the rate of the alcohol corresponding to the map selected by the map switch, and an injector that injects the fuel based on the calculated fuel injection amount, wherein the apparatus comprises: a load determiner that determines whether the load connected to the engine is kept constant; a fuel injection amount injecting increasing or decreasing unit that increases or decreases the calculated fuel injection amount to be injected, when the load connected to the engine is determined to be kept constant; an air/fuel ratio estimator that determines a minimum value of the throttle opening using the change of the throttle opening changed by the throttle opening regulator in accordance with the increase or decrease of the calculated fuel injection amount and estimates an air/fuel ratio at which an output of the engine becomes maximum based on the determined minimum value; a fuel injection amount corrector that calculates a correction coefficient based on the estimated air/fuel ratio, corrects the map by multiplying each fuel injection amount in the map corresponding to the selected position of the map switch by the calculated correction coefficient and corrects the fuel injection amount in accordance with the corrected map; and a correction coefficient storing unit that stores the calculated correction coefficient, wherein the fuel injection amount corrector determines whether the map switch has been switched before starting the engine, and corrects the map using the correction coefficient stored in the correction coefficient storing unit when the map switch has not been switched before starting the engine, whereas the fuel injection amount corrector sets the correction coefficient stored in the correction coefficient storing unit to 1 to initialize the correction coefficient when the map switch has been switched before starting the engine.
 2. The apparatus according to claim 1, wherein the fuel injection amount corrector calculates the correction coefficient based on a ratio of the calculated fuel injection amount to a fuel injection amount at which the throttle opening is minimum and corrects the fuel injection amount by the coefficient.
 3. The apparatus according to claim 1, further including: an informing device that informs an operator of a fact that the fuel injection amount is corrected.
 4. The apparatus according to claim 1, wherein the load determiner determines that the load connected to the engine is kept constant when the detected engine speed is within a predetermined range set based on the desired engine speed continuously for a predetermined time period.
 5. The apparatus according to claim 1, wherein the load determiner determines that the load connected to the engine is kept constant when the regulated throttle opening is equal to or less than a predetermined opening and a change amount of the regulated throttle opening is within a predetermined change amount continuously for a predetermined time period.
 6. The apparatus according to claim 1, wherein the load comprises at least one of a generator, a lawnmower and a snowplow.
 7. The apparatus according to claim 3, wherein the informing device comprises at least one of a lamp and a buzzer.
 8. A method for controlling an air/fuel ratio of a general-purpose internal combustion engine that is supplied with mixed fuel containing alcohol and gasoline and stored in a fuel tank and that is connectable to a load, comprising a desired engine speed input switch adapted for an operator to input a desired speed of the engine, an engine speed detector that detects a speed of the engine, a throttle opening regulator that regulates an opening of a throttle valve installed in an air intake pipe of the engine such that the detected engine speed converges to the inputted desired engine speed, a map switch that selects one of a plurality of maps corresponding to respective rates of alcohol contained in the mixed fuel stored in the fuel tank, the respective rates of alcohol being greater than 0%, the map switch having a plurality of selectable positions corresponding to the respective rates of alcohol and selecting the one of the plurality of maps in response to a selected position, a fuel injection amount calculator that calculates a fuel injection amount of the engine based on the detected engine speed and the regulated throttle opening in accordance with the rate of the alcohol corresponding to the map selected by the map switch, and an injector that injects the fuel based on the calculated fuel injection amount, wherein the method comprises the steps of: determining whether the load connected to the engine is kept constant; increasing or decreasing the calculated fuel injection amount to be injected when the load connected to the engine is determined to be kept constant; determining a minimum value of the throttle opening using the change of the throttle opening changed by the throttle opening regulator in accordance with the increase or decrease of the calculated fuel injection amount and estimating an air/fuel ratio at which an output of the engine becomes maximum based on the determined minimum value; and calculating a correction coefficient based on the estimated air/fuel ratio, correcting the map by multiplying each fuel injection amount in the map corresponding to the selected position of the map switch by the calculated correction coefficient and correcting the fuel injection amount in accordance with the corrected map; and storing the calculated correction coefficient, wherein the step of calculating correction coefficient determines whether the map switch has been switched before starting the engine, and corrects the map using the stored correction coefficient when the map switch has not been switched before starting the engine, whereas sets the stored correction coefficient to 1 to initialize the correction coefficient when the map switch has been switched before starting the engine.
 9. The method according to claim 8, wherein the step of fuel injection amount correcting calculates the correction coefficient based on a ratio of the calculated fuel injection amount to a fuel injection amount at which the throttle opening is minimum and corrects the fuel injection amount by the coefficient.
 10. The method according to claim 8, further including the step of: informing an operator of a fact that the fuel injection amount is corrected.
 11. The method according to claim 8, wherein the step of load determining determines that the load connected to the engine is kept constant when the detected engine speed is within a predetermined range set based on the desired engine speed continuously for a predetermined time period.
 12. The method according to claim 8, wherein the step of load determining determines that the load connected to the engine is kept constant when the regulated throttle opening is equal to or less than a predetermined opening and a change amount of the regulated throttle opening is within a predetermined change amount continuously for a predetermined time period.
 13. The method according to claim 8, wherein the load comprises at least one of a generator, a lawnmower and a snowplow.
 14. The method according to claim 10, wherein the step of informing is made using at least one of a lamp and a buzzer.
 15. The apparatus according to claim 1, wherein the fuel injection amount corrector sets the correction coefficient stored in the correction coefficient storing unit to 1 to initialize when the engine is not started after the corrected fuel injection amount is injected.
 16. The method according to claim 8, wherein the step of calculating the correction coefficient sets the correction coefficient stored in the stored correction coefficient to 1 to initialize when the engine is not started after the corrected fuel injection amount is injected.
 17. The apparatus according to claim 1 further comprising an alcohol rate estimator that estimates a rate of the alcohol in the mixed fuel by using the calculated correction coefficient and the selected position of the map switch.
 18. The method according to claim 8 further comprising the step of estimating a rate of the alcohol in the mixed fuel by using the calculated correction coefficient and the selected position of the map switch. 